It needs to be a qdev property, because it belongs to the drive's
guest part.  Precedence: commit a0fef654 and 6ced55a5.

Bonus: info qtree now shows the serial number.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>